The present invention relates to gas turbine engines for aircraft and, more particularly, to gas turbine engines each coupled to a corresponding auxiliary engine.
Gas turbine engines as continuous combustion, open Brayton cycle internal combustion engines have come to dominate as the power plants for larger, faster aircraft to essentially the exclusion of reciprocating engines, or internal, intermittent combustion engines, earlier used as power plants for these kinds of aircraft. This is largely because of the greater power-to-weight ratio of gas turbine engines versus internal combustion engines, especially in large horsepower engines, or, more appropriately, large thrust engines in which those large thrusts are provided with a relatively small, and so smaller drag, frontal area engine structures relative to reciprocating engines. Gas turbine engines generate such large thrusts for propulsion, or horsepower for engines with an output shaft, by combining large volumes of air with large amounts of fuel, and thereby form a jet of large velocity leading to the capability to provide desired speedy flights.
In addition to providing thrust, such gas turbine engines have operated integrated drive generators to generate electricity for the aircraft and for the engine electronic controls. The amount of electricity needed for these purposes in the past has tended to be relatively modest typically in the range of a few hundred kilowatts of electrical power but, with recently arriving new aircraft, exceeding a megawatt of power. However, there are some aircraft, usually for military uses, that have come to have needs for still larger amounts of electrical power either on a relative basis, the electrical power needed relative to the capability of the gas turbine engine available, or on an absolute basis with power needs significantly exceeding a megawatt. Furthermore, such demands for electrical power in military aircraft often occur at relatively high altitudes and often occur unevenly over relatively long time durations of use, that is, large peaks repeatedly occur in electrical power demand in the course of those long use durations.
Corresponding attempts to obtain the added power from the typical aircraft propulsive system, the gas turbine engine, that are needed to operate the concomitant much larger capacity electrical generators, either on a relative or absolute basis, will subtract significantly from the thrust output of the available turbine or turbines. Making up that thrust loss in these circumstances by operating such available turbine engines so as to increase the thrust output thereof causes the already relatively low fuel use efficiency during flight to decrease significantly which can severely limit the length of otherwise long duration uses, and also brings those engines closer to becoming operationally unstable.
Of course, such problems could be avoided by providing a gas turbine engine of greater thrust capability than is needed to propel the aircraft to thereby provide adequate capacity for generating needed electrical power. However, if that needed power is not needed on a fairly constant basis to propel the aircraft, such a gas turbine engine will often be operating very far from its optimal operating point as it propels the aircraft, and this leads to significantly degraded fuel efficiency thereby limiting the duration of flight in an aircraft using such an engine.
Another possibility is the addition of a further small auxiliary gas turbine engine for operating the electrical power generator to provide the needed electrical power. Again, however, for uneven electrical power demands over time, the engine will either be have to be oversized or will be operated inefficiently in meeting the power peak demands and with operating excursions bringing it near to becoming unstable. Even more important, this small auxiliary engine will have poor fuel consumption because the fuel consumption performance of all turbomachinery worsens as it is scaled down since high loss boundary layer flows and tip clearance flows represent a larger fraction of the total fluid flow of the smaller engine.
In the situation of a relatively small aircraft being selected with a small gas turbine engine for propulsion to provide relatively good fuel consumption at altitude and so long flight endurance, the aircraft would need to have a large wing area and high lift devices, and would have to be operated from long runways. However, the endurance problem is still not satisfactorily solved because such engines do not scale down to small sizes so as to maintain the fuel use efficiencies for power generation and, of course, are limited by instability of the compressor in the peaks of electrical power they can supply.
From the foregoing, gas turbine engines used to propel aircraft can be seen to not be well suited as energy sources for also operating electrical power generators that are required to deliver large quantities of electrical power, especially in situations in which the quantities demanded of that power change substantially over time or in which those quantities must be delivered over long time durations, or both. A more effective alternative is to use a different kind of engine to serve as the energy source for such electrical power generators that is both substantially more fuel efficient and operates stably over a wide range of output powers. Thus, intermittent combustion internal combustion engines can be used, such as gasoline engines operating on the any of the Diesel, Miller, Otto or Wankel cycles.
Such engines can operate with a fuel efficiency on the order of seventy percent (70%) better than that of a continuous combustion (Brayton cycle) internal combustion gas turbine engine. Furthermore, this better fuel efficiency of these engines is essentially maintained over a wide range of engine output powers in contrast to the significant fuel efficiency decreases that occur in gas turbine engines if operated away from their optimum operating points. Although such intermittent combustion engines are heavy relative to the output power they provide, they can be relatively small if used primarily for energizing electrical power generators in an aircraft, rather than for propulsion of that aircraft, and can thus be positioned in the aft part of the fuselage past the cabin pressure bulkhead where, typically, auxiliary power units are housed in commercial airliners.
At high altitudes, however, internal combustion engines of all kinds face the possibility of limited power output because of the relatively small air pressures there limiting the number of chemical reactions of oxygen with hydrogen and oxygen with carbon in the burning the engine fuel in the engine combustion chamber or chambers. Gas turbine engines do provide therein very large air flows through use therein, typically, of axial flow compressors to supply compressed air to the subsequent combustor sufficient to sustain the desired combustion process therein. However, such compressors can provide considerably more compressed air than the minimum needed for this purpose thereby allowing some of this compressed air to be delivered through an air transport duct to the air intake of an intermittent combustion internal combustion engine so that, in effect, the compressors of the gas turbine engine serve as a very capable supercharger for that intermittent combustion engine. The usual mounting positions for propulsion gas turbine engines on passenger or cargo aircraft, at least larger ones, are typically under the wings. This arrangement would thereby require an air transport duct to extend from the compressors therein to the aft part of the aircraft fuselage to supply compressed air to an intermittent combustion engine located there which would be both expensive and require aircraft design compromises. Thus, there is a desire to find a more effective and economical source of pressurized air for operating an intermittent combustion internal combustion engine in an aircraft.